1. Field of the Invention
This invention relates generally to devices for measuring fluid flow rate within a conduit and without direct fluid contact, and more particularly relates to devices for nonintrusively measuring the rate of flow of intravenous fluid or infusate being dispensed by an infusion pump or controller system, as well as the rate of fluid flow in a variety of medical and industrial applications.
2. Description of Related Art
In many applications, the metering of the rate of fluid flow through a conduit is essential, and thus numerous mechanical and electrical means of such flow rate measurement have been developed. Accurate and inexpensive fluid flow measurement without direct fluid contact is especially important in the medical area, where sterilization, safety, disposability of the fluid path, and cost are of great concern. There are other non-medical applications, for example in the chemical process industry, where inexpensive and/or disposable fluid conduits may be required, or where fluids may be present at high temperature or high pressure, or where fluids which are highly caustic or highly toxic may be involved. Many existing types of systems for fluid flow measurement, such as turbine, target, positive displacement, differential pressure, hot-wire anemometers, rotameters, and others, generally incorporate components which must be in direct contact with the flowing fluid. Electromagnetic flow meters based on the Faraday principle require a pair of electrodes in direct contact with an electrically conductive fluid and exhibit poor accuracy below about 100 cc/hr. Other flow rate measurement systems, such as laser Doppler velocimeters and coriolis flow meters, are prohibitively expensive, and still other systems cannot operate over the wide range of rates and fluid types required in many applications. In situations where the fluid's viscosity is unknown, differential pressure types can be inaccurate, and in situations where the fluid's specific heat is unknown, systems which involve the continuous addition of heat can be inaccurate. Low flow rates are very important in numerous industrial and medical applications and are especially challenging for all of the above-mentioned flow measurement techniques.
In the context of administration of fluid infusate in a hospital setting, it is important that the fluid path be disposable and inexpensive and that many different types of fluids be delivered at an accurate flow rate under a variety of conditions. Many medical fluid delivery systems incorporate no specific means of measuring fluid flow rate, relying on the accuracy of a pumping mechanism, and many incorporate a means to measure flow which is highly inaccurate and used only to detect instrument malfunctions. A large number of medical infusion systems measure the rate of flow by counting drops of fluid as they pass through a drip chamber, and others measure flow rate by positive displacement, repeatedly filling a chamber of known volume and dumping its contents by means of an arrangement of valves.
The common method of measuring the rate of flow of administration of an infusate to a patient by counting drops is inaccurate for many reasons. Among these reasons are variation in drop size due to temperature and fluid type changes, surface tension variations, and vibrations. Systems which measure fluid flow by positive displacement require comparatively costly disposable sets due to the close dimensional tolerances necessary in the manufacture of the chamber and associated valves. Furthermore, high viscosity fluids, variations in head height, and in-line filters can all reduce the accuracy of conventional methods of measuring flow rate through a fluid infusion set. In the context of administration of fluid infusate in a hospital setting, it is important that the fluid infusion sets be disposable, and that the infusate administration system should be simple and inexpensive, and accurate with a variety of fluid types over the full range of environmental conditions.
In connection with infusion pumps, drop sensor monitoring generally is used to detect gross errors in flow rate, and is not used to directly control a pumping mechanism. At any given desired flow rate, the pumping mechanism operates in an unchanging manner, independent of head height, fluid viscosity, or pressure, provided that an alarm condition is not triggered in some way.
U.S. Pat. No. 4,491,024 (Miller, Jr.) relates to a method and apparatus for metering fluid flow at rates below 10 cc per minute. A heat pulse is introduced through a calibrated cell and the pulse is detected downstream. The detection of the pulse is used to trigger a subsequent heat pulse, and the time intervals between successive pulses is measured to determine the rate of travel. U.S. Pat. No. 4,532,811 (Miller, Jr., et al) describes a similar apparatus, in which a resistance heating element and a heat sensing thermistor are used with a flow cell for determining rate of flow. U.S. Pat. No. 4,628,743 (Miller, Jr., et al) describes another similar method, in which the time derivative of the heat detector's signal is used to trigger the heat pulses.
U.S. Pat. No. 4,458,709 (Springer) relates to measurement and control of fluid flow rates, and also generation and detection of heat pulses in a liquid flow. A thermal pulse is delivered into the fluid flow, the time of flight of the leading edge of the pulse is detected to determine flow rate, and the lagging edge of the heat pulse is detected, triggering the generation of a subsequent thermal pulse. A thermocouple type sensor is described.
U.S. Pat. No. 4,384,578 (Winkler) describes a thermal fluid flow rate measurement system utilizing continuous external heating and temperature sensing. In that system, the heat required to maintain a temperature differential between a heated area of tubing and an unheated area of tubing is measured as a function of fluid flow in the line. Contact shells are provided around the tube for providing heat to the fluid in the line and for sensing heat. U.S. Pat. No. 4,255,968 (Harpster), describes a flow rate measurement system involving continuous heating and two downstream sensors. A first sensor is placed near the heater within the thermal influence of the heater, and a second sensor is thermally isolated from the heater. The heating element is a resistance heater, mounted with one of the sensors on a thermally conductive conduit forming a portion of the line in which fluid flow is measured. Measurement of fluid flow involves comparing the signals from the first and second sensors to derive the rate of flow.
Another type of externallized sensor is described in U.S. Pat. No. 4,014,206 (Taylor), in which an apparatus and method for monitoring air emboli in tubing carrying blood or other fluid, such as during kidney dialysis, involves the measurement of the complex dielectric constant of the fluid, without direct fluid contact, to monitor changes in the fluid which may be caused by passage of air in the tubing. U.S. Pat. No. 4,234,844 (Yukl) also describes a method and apparatus for following changes in the complex dielectric constant of fluid within a region of interest, such as within the heart wall in a person's body, without directly contacting the fluid being monitored.
It would be desirable to provide a flow measurement means allowing for the design of fluid delivery systems which are "closed loop", in which information from the flow measurement means is used to control the pump mechanism. In this way, the system can dynamically correct for any condition which would alter the flow rate, such as head height or pressure; and accuracy constraints on the pumping mechanism itself are significantly reduced. It would be desirable to provide an accurate means of fluid flow measurement which does not involve direct fluid contact in medical applications where toxicity or potential for infection is significant, such as in the metering of urine output or blood. Additionally, it would be beneficial in many medical and industrial applications to provide a flow rate measurement system which can function accurately without direct fluid contact over a wide range of flow rates, fluid types and environmental conditions. When applied to flow measurement systems which are based on the generation and detection of heat pulses, resistance heaters and thermistor or thermocouple sensors generally must be in contact with the fluid and also involve time lags due to heat convection, which affect the precision of measurement. It would also be desirable to provide a mode of heating and sensing which did not involve convection time lag.
It would further be desirable to provide a disposable, inexpensive, accurate system of measuring flow of fluid, utilizing precise and rapid generation and sensing of heat pulses, neither requiring contact with the fluid, nor individual calibration of each set, and which can work accurately with a variety of fluids regardless of specific heat or viscosity. The present invention meets these needs.